Computer Networks · free preview
Ethernet and Collision Detection
The Ethernet is the canonical multi-access link, a shared medium where every adaptor hears every frame and the access algorithm decides who may speak, its frames carry a synchronizing preamble, globally unique 48-bit addresses, a demultiplexing type field and a 32-bit CRC, its transmitter is 1-persistent and detects collisions as it sends, the worst-case collision can take a full round trip to surface and so frames are padded to 512 bits, and repeated collisions are defused by exponential backoff, with modern switched Ethernet retiring the algorithm while its descendant lives on in wireless.
1. A Shared Medium
A Shared Medium
2. The Transmitter Algorithm
The Transmitter Algorithm
3. The Worst Case
The Worst Case
4. Exponential Backoff
Exponential Backoff
5. Hearing every frame
Why does an Ethernet adaptor need a destination address check at all, given that frames are delivered point to point on modern links?
6. The 1-persistent choice
An Ethernet adaptor has a frame queued and senses the line busy. What does the 1-persistent algorithm do when the line goes idle?
7. The jamming sequence
Upon detecting a collision, why does an adaptor transmit a 32-bit jamming sequence instead of simply stopping?
8. The worst-case window
Host A transmits at time t on a maximal Ethernet with one-way latency d. Host B starts at t+d, just before A's signal arrives. When does A learn of the collision, and what follows?
9. Why randomize
After two stations collide, why does each pick its retry delay at random from an interval that doubles per collision, rather than waiting a fixed slot?
10. Switched Ethernet today
Why does the collision detection algorithm matter little on a modern wired Ethernet, yet still matter to this course?
11. Minimum frame size
Every Ethernet frame must be at least how many bits long, so that a sender still transmitting can detect the worst-case collision?
12. The jam
How many bits long is the jamming sequence a transmitter sends after detecting a collision?
13. Interframe gap
Adaptors wait a fixed quiet interval after a frame ends before the next may begin. How long is this interframe gap in microseconds?
14. Backoff ceiling
Under exponential backoff, after how many failed retransmission attempts does an adaptor give up and report an error to its host?
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